Add spec for Purge (#25)

spec/cli/commands/purge_spec.rb

@@ -4,4 +4,18 @@
 
 describe Ronin::Wordlists::CLI::Commands::Purge do
   include_examples "man_page"
+
+  describe '#run' do
+    let(:cache_dir) { instance_double('CacheDir') }
+
+    before do
+      allow(Ronin::Wordlists::CacheDir).to receive(:new).and_return(cache_dir)
+    end
+
+    it 'must purge all cached wordlists' do
+      expect(cache_dir).to receive(:purge)
+
+      subject.run
+    end
+  end
 end